One of the first things I discover under the tutelage of my Equinox trainers is that I've not paid nearly enough attention to my core. That vaguely defined network of muscles – roughly from below the chest to the tops of the knees – is nothing short of almighty. It is every bit as essential to functional fitness as its name implies. My trainers – Equinox training manager Vural Vural (who goes by the more succinct "V") and trainer David Juhn – have a deep and abiding respect for the core.

Equinox subjects new training recruits to a test. The Functional Movement Screen (the FMS) was developed by physical therapist-strength trainer Gray Cook, who sought out a uniform way to assess fitness beyond strength and flexibility. The FMS [see the Assessment: How Well Can You Move?, p. 98] involves few movements you typically see at a gym, and nothing you'd glimpse on Muscle Beach. Instead, it features simple stepping actions, basic squats, some rotated lunges, and one seemingly impossible motion that asks you to get on your hands and knees and then lift your right leg and right arm off the floor simultaneously.

None of this goes well. Of a possible 21, I score a 12. The number matters less than the dysfunction it represents: According to V, I am a casualty of the modern office environment – hunched, misaligned, and, having let some important muscle chains go fallow. In anatomical terms, I lack the all-important core competency, and my adductors – a group of muscles in the thigh – are dominant, as illustrated by the way my knees flare during the squat. Most important, I suffer from what V calls "glute amnesia," which not only contributes to my knee issues but also is kryptonite to core competency. "You spend so much time in a seated position at the office that you forget how to engage your glutes" [see Working-Stiff Workout, next page].

He has another name for the condition – "dumb ass" – but no matter what we call it, it's clear I'll get nowhere if my glutes and abdominal wall don't form a chain of musculature that is able to transfer power from my lower girdle (hips) to my upper girdle (shoulders), and from side to side. It's a key transaction that makes everything else possible. "Your core should be firing first on any activity you do," he tells me. "It's imperative to everything you do." After the first week – I train with each of the trainers once a week – I question whether I'm even sitting or climbing stairs correctly. Actually, I know the answer: I'm not.

For the next few weeks, no one asks me to do a crunch or a curl or a bench press. Most of the work we do uses my own body weight, an occasional medicine ball to squeeze between my knees (glute activation!), and some machines to improve rotational mobility in my torso. Vural keeps me on a strict diet of core activation. So while hunks of iron are being hoisted and swung around me, I lie on my back thrusting my hips in the air while V pokes at my butt and abs. I am an island of Jazzercise in an ocean of 300. V and I rarely wander in the more gratifyingly macho gym material – that's more the realm of David, who has me do pull-ups and such, but only after squats and low-to-high cable pulls for my torso. They are both sticklers for form and speak more like orthopedists than trainers.

It takes just a couple of weeks for me to shed some pounds and feel a slight shift in the age-old metabolic tug of war between fat and muscle. The results are mostly interior and invisible, but there is a new solidity forming beneath the surface. Walking feels different, with more awareness in my hips and better posture, more balance. One morning I reach down to tie my shoes and realize that I've lifted my foot instead. And I do this without falling over.

The twice-weekly sessions are challenging, with sets often ending in fatigue, but they're hardly sadistic. Though praise is hard to come by, there's little of the macho shaming that makes high school football such a misery. After one less-than-exhausting workout, I ask V when we're ready to increase the load. "You're not ready," he explains. "Let us build you up first," he tells me, "then we'll destroy you."